1. Field of the Invention
The present invention is directed to a tissue-optical measuring arrangement for the examination of a living subject with visible, NIR or IR light. The wavelength of the visible light lies between 380 and 780 nm, that of NIR light (near infrared light) lies between 780 nm and 1.5 .mu.m and that of IR light (infrared light) lies between 1.5 .mu.m and 1 mm. It is particularly the range from 1.5 .mu.m through 15 .mu.m that is of significance in the present invention given the employment of IR light.
2. Description of the Prior Art
Many optical properties of tissue such as, for example, the absorption, the scattering (dispersion) and the spectral properties can be identified by exposing the tissue to light of the aforementioned wavelength ranges and analyzing the transmitted and/or reflected light. For example, it is possible to identify tissue modifications in mammary diagnostics or to acquire information about the blood supply of the brain in pediatrics and/or neurology by detecting light of the aforementioned wavelength ranges into the respective subject, for example a mammary gland or a skull, detecting the light emerging from the subject and interpreting the information acquired in this way in a suitable way. It is thereby advantageous that these are usually non-invasive procedures. Further details can be derived, for example, from the publications "Cerebral Oxygenation Measuring System NIR-100" (Tentative Data), Hamamatsu Photonics K.K., System Division, September 1987; "INVOS-In Vivo Optical Spectroscopy", Somanetics Corporation, USA; and "Cerebral Monitoring in Newborn Infants by Magnetic Resonance and Near Infrared Spectroscopy", D. T. Delpy et al., Departments of Medical Physics and Bioengineering, Pediatrics and Physiology, University College London.
Unfortunately, the light emerging from the subject that is to be detected in such procedures, which can be back-scattered (diffusely reflected) light or transmitted, dispersed, light contains information about the entire region of the subject illuminated with the incoming light. The measurement is thus not location-selective. This means that one does not known what path the detected light took in the subject and/or cannot determine from what depth of the subject the detected light was reflected. Additionally, in the case of detecting the back-scattered light, most of the light back-scattered from the surface of the subject and the surface-proximate regions thereof must also be measured. This leads to a poor signal-to-noise ratio of the measurement, so that smaller regions which deviate in terms of their optical properties from the surrounding tissue such as, for example, nascent tumors, cannot be recognized. This also applies in the case of detection of the light transmitted through the subject, since the signal-to-noise ratio deteriorates with increasing thickness of the subject to the point that the results are unusable.
Heretofore, essentially only one method has been fundamentally suitable for employment in vivo that offered an incipient solution to these problems. This method, described in the article "Estimation of Optical Pathlength through Tissue from Direct Time of Flight Measurement", D. T. Delpy et al., Phys. Med. Biol., 1988, vol. 33, No. 12, pp. 1433-1442, is based on the "time of flight" measuring principle using a pulsed laser as a light source and an ultra-fast streak camera as the detector means. The pulse duration of the laser is typically less than 1 picosecond. The cronological resolution of the streak camera lies on the order of magnitude of approximately 2 picoseconds. Since the light is back-scattered from the subject to be examined in different depths, or penetrates the subject on different paths, the individual parts of the back-scattered or transmitted light have different arrival times at the streak camera. The detected light parts can thus be selected and detected according to arrival time and, thus, according to the depth in the subject from which they were back-scattered, or the path which they took through the subject. A time of flight measuring system having an adequate chronological resolution and, thus, an adequate topical resolution, however, is expensive.